We propose to continue the operation of a national stable isotope resource (SIR) at the Los Alamos National Laboratory. The principal goal of the SIR is to promote new applications for the isotopes of carbon (12C, 13C0, oxygen (12O, 17O, 18O), nitrogen (14N, 15N), sulfur (32S, 33S, 34S, 36S), and to a lesser degree selenium (77Se) in solving fundamental problems in biochemistry, biophysics, as well as practical problems in medical science. The research component of the proposal suggests development of efficient procedures for specific labeling of amino acids, nucleic acids, enzyme cofactors, and new starting materials for syntheses. By making these materials available to the research community, the Resource will contribute to fundamental studies of protein and nucleic acid structure, protein-nucleic acid interactions, and structure-function relations in heme proteins. The Resource will continue to carry out a spectrum of core, service, collaborative, and training activities aimed at broadening the uses for stable isotopes in biomedical research. It will continue its contribution to the developing commerce in stable isotopically labeled compounds by devising efficient procedures for converting labeled CO and NO to useful materials. Particular compounds that become the focus of Resource activities will be selected in consultation with the Resource's Advisory Committee and will be provided to external investigators on a competitive basis. The latter is one of the most important services of the Resource.